1. Field of the Invention
The present invention relates to a method of and an apparatus for measuring the outer diameter and the structure of an optical fiber, and more particularly to a method and an apparatus of measuring the outer diameter and structure of an optical fiber without cutting the optical fiber being examined.
2. Prior Art
Conventionally, there has been employed a method of and apparatus for measuring the outer diameter and the internal structure of an optical fiber by cutting the optical fiber being examined and by observing the cross section thereof through a microscope or television camera.
FIG. 1 shows a conventional arrangement for measuring the outer diameter and the internal structure of an optical fiber by means of a microscope, wherein light from a light source 1 is first converged by an injection lens 2 and caused to be incident on an end of an optical fiber 3.
The light from the light source 1 thus converged is propagated through the core of the optical fiber 3. However, the light in a clad of the optical fiber is attenuated during the propagation thereof through the clad because it is radiated from and absorbed by the clad. As a result, a light emitted from an exit edge 3a to be observed originates in only the light propagating through the core, and therefore is used as an illuminating light for the core.
On the other hand, as a light from a light source 10a is incident on the exit edge 3a being observed using a half mirror 9a and is reflected from the clad of the exit edge 3a, it is used as an illuminating light for the clad.
In the rear of the exit edge (edge being observed) 3a , a pickup tube 4 is located so that the edge 3a of the optical fiber can be observed with a lens 8a. The pickup tube 4 is connected to a TV monitor 5a, which is capable of magnifying and displaying an image (light-intensity distribution) of the edge 3a being observed. The luminance distribution on a line A-A' of the monitor TV is as shown in FIG. 2.
The conventional method and apparatus as described above has the following disadvantages.
In the first place, an optical fiber being examined is cut and the cross section thereof is used to observe its structural parameters under the conventional cutting method and accordingly the optical fiber being examined is infallibly damaged;
Secondly, because only the section of an optical fiber can be observed at the cross section thereof through the conventional method, it is impossible to measure the structural parameters continuously along the optical axis of the optical fiber. For the above reason, changes in the structural parameters produced locally in part of the optical fiber are hardly intended for measurements, whereby the total length of the optical fiber cannot be measured accurately;
Thirdly, although a high level of technique is required for the cutting operation, scratches and breakages produced during the cutting operation of an optical fiber allow the inclination of the cross section thereof, which results in the reduction of measurement accuracy. Consequently, the structural parameters are hardly measured accurately;
Fourthly, the section of the optical fiber thus inclined wears a tilt angle relative to the optical axis thereof and, in this case, the difference in observing magnification between the upper and lower portions (or left- and right-hand sides) of the monitor TV 5 makes difficult the caliberation of absolute values; and
Fifthly, as shown in FIG. 2, (i) and (iv) portions in the light-intensity distribution at the edge of the optical fiber are not completely in step form due to the characteristics of the pickup tube and tilting to some extent. Consequently, measurement must be made by presetting the threshold level to recognize (i), (iv). However, since the (i) and (iv) portions in the light-intensity distribution are dependent on the characteristics (.gamma.-characteristic, etc.) of the element for use and an illuminating light, it is hardly possible to accurately measure the outer diameter and internal structure of the optical fiber.